Food waste is generated at a phenomenal scale around the world. The rate at which this waste accumulates and its putrescible and polluting nature represents a problem from economic, social and environmental standpoints.1 Waste treatment is highly regulated and leads to significant costs. Transformation of waste into high value-added products allows companies to reduce treatment costs, generate additional profits and thus improves their competitiveness. Moreover, the recovery and valorisation process of by-products is part of existing sustainable development and environmental protection requirements.2 
Fruit and vegetable wastes are produced mainly as the result of processing.3 Citrus processing waste (CPW) represents a highly relevant feedstock with a great potential in the development of a resource focused biorefinery. CPW is the solid by-product predominantly obtained after juicing operations.4 Citrus are the largest fruit crop in the world with 94.8 million tonnes grown in 2005.5 They include mainly: oranges, lemons, limes, grapefruits and tangerines. The largest citrus producing countries, accounting for more than 70% of the world's supply, include the USA, Brazil, China, India, Japan, Spain, Italy, Egypt, South Africa, Turkey and Morocco, with Brazil leading the world in citrus production.5 As part of the citrus market, processing for juice and other products is very important. Over a quarter of the total citrus production in 2005 has been processed.6 Processing creates a large amount of waste by-product in the form of peel, seeds, rag (the membranes between the citrus segments) and pulp (juice sacs), representing ca. 50-60% of the whole fruit being discarded after juicing for example.6 Orange juice production generates waste on a multi tonne scale worldwide: Florida state produces an estimated 5 million tonnes of CPW every year.7 The Mediterranean region produces 20% of the total citrus fruit production, with Spain being the main producer.8 
Citrus waste presents major environmental problems as its high carbohydrate content is highly fermentable.1 Major components of wet CPW are water (80%), soluble monosaccharides, cellulose and hemi-cellulose, pectin, limonene, flavonoids and proteins.9 CPW can be processed to remove and evaporate the free liquid, dried and be sold as cattle feed, since the waste peel provide filler and a protein source. But with a protein content of only 6%, it is not a high protein source.10 Drying CPW (to 10% moisture) is very energy-intensive and costly due to the high water content7 and using CPW as cattle feed is only marginally profitable.
In recent years, valorisation of CPW has received increasing interest due to the wide variety of interesting compounds it contains. D-limonene (3.78% of dry weight of CPW)11,12 is an attractive starting compound for industrially relevant fine chemicals and flavour compounds (with identical carbon skeletons, such as carveol, carvone, α-terpineol, perrillyl alcohol and perillic acid).1 D-limonene can be extracted from citrus peels by steam distillation, extracting over 90% of the limonene this way.13 Steam diffusion techniques using a steam generator (i.e. using extra water and extra energy to remove this water) result in 1.54% yield (w/w) of essential oil from fresh orange peel, and process time can be reduced to one quarter with the use of microwave irradiation.14 Ultrasonic methods have also been used for peel treatment increasing yield of essential oils.15 
Another valuable component of CPW is pectin. Pectin, a complex structural hetero polysaccharide found in non-woody plant tissues, is an important food additive mainly used as a gelling agent and a thickener. It is also used in skincare cosmetics and as drug substrate for colonic drugs. Wet or dry citrus fruits contain roughly 20-30% extractable pectin.16 The current extraction process is based on acidic hydrolysis of citrus (or apple peel) using a dilute mineral acid (nitric, sulphuric or hydrochloric acid) between 50 and 100° C. and at pH 2-3 for several hours to solubilise the protopectin. Pectin is recovered by precipitation with isopropanol. Common yields of pectin are ˜3% of the peel weight.17 Enzymatic methods have also been studied for pectin extraction, avoiding the reduction of the degree of polymerisation linked with uncontrollable acid hydrolysis conditions.18 
CPW has other applications, such as a source of fibre, flavonoids from the peel (used in the production of human food and food supplements), as a binding agent in foods, a fermentation substrate for single-cell protein production, and as silage and mosquito repellent.1 A significant body of literature is available on valorisation strategies for CPW. CPW has been used for pectin extraction by acid hydrolysis and production of activated carbon,19 pectic enzyme production,5 dietary fibre extraction,20 methane production,21 single-cell protein production,22 bio-ethanol production by a variety of microorganisms and including simultaneous saccharification and fermentation 4,5,9 and succinic acid production.13
However, these applications are limited, especially when considering the volume of CPW and its interesting chemical content.
Nevertheless, there has been little industrial uptake yet and no integrated use of CPW close to the major sources of production, apart from efforts to provide a process for the recovery of a wide variety of products (monosaccharides, essential oils, bio-flavonoids and solid mixture of polysaccharides) suitable for industrial applications. However, in this case the major components of orange peel (cellulose and pectin) have not been separated.23 Likewise, low grade pectin-cellulose mixture is the major product of another process.24 
Hydrothermal and microwave processing are known to be effective techniques to break down and extract from bio-waste25 and specifically for citrus peel.26 Microwaves are more energy efficient and safer for heating25,27 and allow the whole medium to be heated simultaneously unlike with conventional heating with which heat is only introduced at the interface of the sample and the heater.28-30 Microwave processing has been shown to be effective at pilot scale and at large continuous processing scale, for example in waste treatment.31 The use of microwave irradiation for the conversion of biomass to valuable products has a number of important advantages:                (i) it is mobile;        (ii) it is flexible;        (iii) it reduces CO2 burden;        (iv) it is rapid;        (v) it can be continuous; and        (vi) it has high energy efficiency.        
Microwave processing has previously been shown to be effective in the breakdown of citrus peel, enabling extraction of individual phytochemicals like hesperidin, limonene 2,14,32,33 and pectin.34-36 
A number of reports show that microwave assisted extraction yields pectin of higher quality due to the rapid heating, breaking fewer covalent bonds. 35,37,38 Nevertheless, all these methods use microwave heating as a method of pre-treatment and the pectin is still extracted using acid hydrolysis:                (i) In Kratchanova's work, microwaves were used just as a pre-treating method and the pectin was then extracted by at 80-82° C. with HCl (0.5 M) to lower the pH to 1.5.40;        (ii) Zhongdong et al. uses the microwaves as a heating method for the extraction of pectin at pH 2 with HCl (85° C. for 4 minutes, 2450 MHz, 1000 W)36; and        (iii) Fishman et al. extracts pectin from the albedo of oranges using HCl as a solvent (albedo:solvent 1:25).39,40,37         
The production of pectin is especially resource demanding and wasteful since it produces large quantities of acidic wastewater.
However, a microwave assisted citrus peel biorefinery, which processes major components of CWP (pectin, cellulose, limonene, monosaccharides, etc.) all together and minimises waste, and without pre-treatment or added acid, has not yet been developed.